I think of giratina as a god, so i gave it a random moveset
Giratina
Nature: Serious
Item: Griseous Orb
Ability: Levitate
Form: Do I even have to say?
- ThunderBolt (Good Damage, May paralyze)
- Psychic (May lower SP.Defense)
- Shadow Claw (Type Coverage, Hi-Critical Ratio)
- Shadow Force (High Damage, Type Coverage, Also Stops Protect or Detect)
This all started from teaching giratina thunder for no reason.
Sorry for all of the info too :P.

And if you really wanted a special rock type move, it does learn Ancient Power. 10% chance of lowering Sp.D, or 10% chance of raising all stats? Although, 5 less PP and 30 less power. Aura Sphere isn't very good against flying and bug types. Flying types are usually paired with normal, so Shadow Ball becomes useless. But rock can fend off flying and bug types, which are combine commonly.

Shadow Claw (STAB, used to hit other Ghosts.)
Outrage (STAB, deals with other Dragons)
Stone Edge (Used to deal with Ice-types, along with coverage against Bug, Fire, and Flying-types.)
Silver Wind (Weak Base Power, but it may boost all stats. Also provides coverage against Grass, Dark, and the rare Psychic-type with good Defense.)

Giratina looks suspiciously like a Ghost/Steel type.
You cannot tell me it doesn't have like steel claws.

Shadow Force has an amazing base power of 216 including STAB and the Griseous Orb Boost.
Hearing some poeple prefer Shadow Claw/Ball over Shadow Force because afraid of a Normal type switching in, The only Normal types that might appear in the uber tier are Arceus and Blissey, Blissey cannot handle a STAB Griseous Orb Boosted Outrage, So that's not a problem.
Normal Arceus is more of a problem, But has immunity to all Ghost attacks anyway.
Outrage is also STAB and boosted further by Griseous Orb, 216 Base Power as well.
Shadow Sneak for some nice Priority being STAB. It has 72 base power with STAB and Griseous Orb, Which is good for a Priority move.
Will-O-Wisp can cripple something, Giving Giratina a nice Chance to use one of its attacks. Dragon Tail racks up some entry hazard and stops opponents who wants to set up.
[1]: http://www.gtsplus.net/files/images/art/hires/487origin.png
[2]: http://www.gtsplus.net/files/images/sprites/plat2fs/487origin.png

NOTE- In the Origin Forme, you may not want a Modest or Adamant nature, seeing as how Giratina is an awesome mixed attacker, doing decent damage in both move categories.
In the Alternate Forme, however, Giratina is a tank, so a nature that boosts Defense or Special Defense but lowers Speed will help you a lot, seeing as how Giratina is fairly slow.

Sub-Split gives this bulky monster a nice sub to let him phase and share his pain whenever he gets low. Dragon tail is STAB phasing and Shadow Sneak finishes off weaker prey. Tis set works best if you have another team mate already set up hazards.

This is a weird set I came up for it and it works great!
Note: This moveset is used when you were previously set up on Spikes and Stealth Rock
Giratina @ Griseous orb
Ability: Levitate
Jolly Nature (+ Speed -Sp Attack)
Evs: 252 Sp Def 252 Def 6 Speed
Role: Annoying bulky wall
Moveset:
- Roar (To switch out defending Pokemon to cause damage)
- Rest (Heal Move to keep the annoyance going!)
- Sleep Talk (You don't want to be completely defenseless!)
- Outrage (High Power, Stab, Pokemon won't get locked in if used in sleep talk)

The plan is to Will-O-Wisp and protect so the opposing Pokemon get hurt and Tina heals. Then with dragon tail to damage and switch out the opponents Pokemon. Works well with entry hazards setters like Deoxys-D.

Earth Power/Earthquake: Both have coverage vs Fire, Electric, Poison, Rock, and Steel. EP: Good Atk, Great Acc, chance to lower Sp.Def. EQ: Great Atk and Acc, hits with 2x Atk, if Dig made the enemy go underground.